Porous PTFE membranes have been conventionally used as air filter media in a variety of fields. Porous PTFE membranes, which have high dust collection performance, are very suitable for use in a place with a little amount of dust (such as use in a clean room). However, when a porous PTFE membrane is used in an intake air filter for an outdoor air conditioner or turbine to filter out air dust, the porous PTFE membrane may collect air-borne dust only in its surface portion and be consequently clogged, causing an increase in pressure drop. An attempt has thus been made to prevent clogging of a porous PTFE membrane and increase the service life of an air filter medium by disposing an air-permeable member such as a non-woven fabric as a prefilter layer on the upstream side in the air flow direction to collect large particles of dust in advance (Patent Literature 1). However, the air filter medium described in Patent Literature 1 has a problem in that the achievement of the preventive effect on clogging of the porous PTFE membrane requires thickening of the prefilter layer, which leads to a high manufacturing cost. There is also a problem in that thickening of the prefilter layer makes pleating (folding into a series of W-shapes) of the air filter medium difficult.
As a product designed to prevent clogging of porous PTFE membranes with dust, there has been proposed an air filter medium in which a first porous PTFE membrane and a second porous PTFE membrane are stacked, the average pore diameter of the second porous PTFE membrane is larger than the average pore diameter of the first porous PTFE membrane, and the second porous PTFE membrane is disposed upstream of the first porous PTFE membrane in an air flow direction (Patent Literature 2). Patent Literature 2 states that the second porous PTFE membrane functions as a prefilter to collect large-diameter particles of dust, thus reducing the increase in the pressure drop across the air filter medium ([0006]). In the air filter medium of Patent Literature 2, the average pore diameters of the porous PTFE membranes are controlled to reduce the increase in pressure drop, as is evident from the fact that the collection efficiency of the air filter medium is measured using polydisperse dioctyl phthalate (DOP) with a particle diameter of 0.1 to 0.2 μm.